Homology-based cloning has revealed six members of the family of interleukin-17 (IL-17) cytokines, which are termed IL-17A to IL-17F. IL-17A, which is produced by the T helper 17 (TH17) subset of CD4+ T cells, is the prototypic IL-17 family member, and it exerts its actions either as a homodimer or as a heterodimer with IL-17F. IL-17 is required for host defense against extracellular microorganisms and is also involved in the pathogenesis of various human and animal autoimmune diseases, such as rheumatoid arthritis, multiple sclerosis (MS), experimental autoimmune encephalomyelitis (EAE, a mouse model of MS), and allergen-induced pulmonary inflammation. The main function of IL-17 is to coordinate local tissue inflammation through the increased production of proinflammatory and neutrophil mobilizing cytokines and chemokines. IL-17 promotes the accumulation of neutrophils in the bronchoalveolar lavage (BAL) fluid of rats and mice. Increased concentrations of IL-17 are found in the lungs and blood of allergic asthma patients and are correlated with the severity of asthma. Kolls et al., Am. J. Respir. Cell Mol. Biol. 28, 9-11 (2003).
Another well-characterized IL-17 family member is IL-17E (also referred to as IL-25), which is the most divergent member of the IL-17 family. IL-25 is produced by airway epithelial cells in response to allergens and by mouse CD4+ T cells that have a TH2 profile, as well as by the human eosinophils and basophils, which are innate effector cells. IL-25 plays a critical role in the initiation and propagation of the TH2-type immune response. Wang et al., J. Exp. Med. 204, 1837-1847 (2007). Transgenic expression of IL-25 in mice as well as recombinant IL-25 induces TH2-type immunity and leads to increases in the concentrations of the TH2-type cytokines IL-4, IL-5, and IL-13, as well as increased eosinophilia and serum concentrations of immunoglobulin E (IgE). Tamachi et al., J. Allergy Clin. Immunol. 118, 606-614 (2006). In IL-25-deficient mice, expulsion of helminth parasites is delayed, indicating an impairment of the TH2-type immune response. Furthermore, endogenous IL-25 is critical in allergen-induced pulmonary airway hyperreactivity (AHR) and inflammation in a mouse model of asthma. Increased concentrations of IL-25 are detected in asthmatic lung tissues compared to those found in normal tissue, highlighting a role for IL-25 in allergic pulmonary inflammation. Angkasekwinai et al., J. Exp. Med. 204, 1509-1517 (2007).
Given studies that have defined critical roles for IL-17 in autoimmune inflammatory responses, a mechanistic understanding of IL-17-mediated signaling is required to develop new and improved therapeutics for the treatment of inflammatory diseases. IL-17R (IL-17RA and IL-17RC) and IL-25R (IL-17RB and IL-17RA) belong to the recently defined family of SEFIR proteins, which are characterized by the presence of a conserved SEFIR domain in their cytoplasmic regions. Novatchkova et al., Trends Biochem. Sci. 28, 226-229 (2003). SEFIR domains share limited homology with Toll-IL-1 receptor (TIR) domains of Toll-like receptors (TLRs) and IL-1Rs, and SEFIR and TIR domains together constitute the (SEFIR/TIR) domain superfamily. Because TIR domains mediate TIR-TIR homotypic interactions, it was thought that the SEFIR domain might also mediate protein-protein interactions.
The inventors and others have reported that the adaptor protein nuclear factor kB (NF-kB) activator 1 (Act1, encoded by the gene TRAF3IP2), which is also known as CIKS [connection to inhibitor of kB kinase (IKK) and stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK)], is a key component in IL-17 signaling. Claudio et al., J. Immunol. 182, 1617-1630 (2009). Qian et al., Nat. Immunol. 8, 247-256 (2007). Act1 also contains a SEFIR domain and is a member of the SEFIR protein family. Upon stimulation with IL-17, Act1 is recruited to the IL-17R and the IL-25R through SEFIR-SEFIR domain interactions, which is followed by the recruitment of transforming growth factor β-activated kinase 1 (TAK1) and tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), which mediate downstream activation of NF-κB. Chang et al., J. Biol. Chem. 281,35603-35607 (2006). In addition, Act1 contains a helix-loop-helix domain at its N terminus, two TRAF-binding sites, and a U-box-like region N-terminal to the SEFIR domain. The inventors found that Act1 functions as an E3 ubiquitin ligase through its U-box-like region and that this activity is essential for IL-17-mediated signaling pathways. Liu et al., Sci. Signal. 2, ra63 (2009). Consistent with its role in signaling, Act1 is required for IL-17-dependent expression of genes encoding proinflammatory factors and for pulmonary neutrophilia, whereas Act1 deficiency abolishes IL-25-induced production of TH2-type cytokines and IL-25-dependent pulmonary eosinophilia. Swaidani et al., J. Immunol. 182, 1631-1640 (2009). However, although heterodimeric interactions between the SEFIR domain of Act1 and that of the IL-17R are known, the molecular details of SEFIR-SEFIR interactions still remain unclear.